The Middle Ages

Ibn Al-Haitham (965-1040) did not accept the theory that objects are seen by rays emanating from the eyes and maintained that light rays originate at the objects of vision. He studied the passage of light through various media and carried out experiments on the refraction of light as it crossed the boundary between two media. He became known as 'the father of modern optics' and was the author of many books — one of the best-known, Kitab Al-Manathr, was translated into Latin in the Middle Ages. It speculated on the physical nature of light, described accurately the various parts of the eye, and was the first to give a scientific explanation of the process of vision. This was a monumental work, based on experiment rather than dogmatism.

Rene Descartes (1596-1650) considered light as a sort of pressure transmitted through a mysterious elastic medium called the ether, which filled all space. The remarkable diversity of colours was attributed to rotary motions of the ether.

Galileo Galilei (1564-1642) developed the experimental method and prepared the way for a proper investigation of the properties of light. The transmission of light had been thought to be instantaneous but Galileo tried to measure the speed of light by putting two people on hills separated by about a mile. One opened a lantern and the other raised his hand when he saw the light. No time difference was detected, which is not surprising since the time interval, based on the currently accepted speed of light, would have been about five microseconds. (There are one million microseconds in one second.)

The law of reflection was known to the ancient Greeks. To put it simply, it says that light is reflected from a surface at an angle which is symmetrically opposite to the angle at which it came in.

Ibn Al-Haitham. Courtesy of The Pakistan Academy of Science.

The law of refraction was discovered experimentally in 1621 by the Dutch mathematician Willebrord Snell (1580-1626). It deals with what happens when light goes from one medium into another. Snell died in 1626, without publishing his result. The first mention of it appeared in the Dioptrique by Rene Descartes, without reference to Snell, but it is generally believed that Descartes had in fact seen Snell's unpublished manuscript.

n1 and n2 are called refractive indices and are properties of the respective media, while the angles d1 and d2 are as indicated in the diagram. Note the bending of the light as it travels from one medium to another.

The laws of reflection and refraction are the basis of the whole of geometrical optics and form the subject matter of Chapters 2 and 3. Both these laws can, in turn, be derived from an even more fundamental law — discovered by the French mathematician Pierre de Fermat (1601-1665) — formulated as the principle of least time. (For a biographical note on Fermat see 'A Historical Interlude' at the end of the next chapter.)